The present invention relates to an exhaust gas recirculation (EGR) control device for controlling the quantity of exhaust gas to be recirculated for an internal combustion engine (IC engine).
A known arrangement is to recirculate exhaust gas by introducing part of the exhaust gas into the intake side of an internal combustion engine in order to reduce NOx as a noxious component contained in the exhaust gas of the engine.
The flow rate of the exhaust gas to be thus recirculated is primarily concerned with reduction of NOx but it also affects the performance of the engine and the fuel cost, so it is desirous to accurately control the flow rate in accordance with the operating conditions of the engine.
FIG. 1 shows a conventional EGR control device of the sort disclosed by Japanese Patent Application (OPI) No. 93950/80. In FIG. 1, there is shown an engine body 1 with an intake manifold 2 and an exhaust manifold 3.
The intake manifold 2 is fitted with a fuel supply unit 4, whereas a throttle valve 5 is disposed close to a connection of the intake manifold 2 and an intake duct 6. An air cleaner 7 is disposed at the inlet of the intake duct 6.
An intake vacuum leading-in passageway 9 communicates with the intake manifold 2 and an intake pressure detector 10 is employed to detect the intake pressure of the intake manifold 2 via the intake negative pressure leading-in passageway 9.
The outputs of the intake pressure detector 10, an opening detector 13 of an EGR control valve 12 and an engine RPM detector 8 are supplied to an EGR control circuit 14.
The EGR control valve 12 is installed in an EGR passageway 11, which communicates with the exhaust manifold 3 and the intake manifold 2.
The output of the EGR control circuit 14 is used to control a control vacuum generator 16. The control vacuum generator 16 generates an actuator vacuum by regulating the intake vacuum and the atmospheric pressure to control the open-close degree of the EGR control valve 12.
The operation of the device will subsequently be described. The engine speed as a volume indicating the operating conditions of the engine and the intake pressure of the engine are respectively detected by the engine RPM detector 8 and the intake pressure detector 10 and the results thus detected are supplied to the EGR control circuit 14.
The volume of the EGR flowing through the EGR passageway 11 is determined by controlling the opening of the EGR control valve 12 by means of the output signal of the EGR control circuit 14 to regulate the output vacuum of the control vacuum generator 16 using the pressures in the intake vacuum leading-in passageway 9 and an atmospheric pressure leading-in passageway 15, so that the comparative deviation of the output value of the opening detector 13 corresponding to a target EGR rate stored in the EGR control circuit 14, depending on the volume indicating the operating conditions of the engine detected by the engine RPM detector 8 and the intake pressure detector 10, from the measured output value of the opening detector 13 interlocking with the EGR control valve 12 becomes zero.
More specifically, the opening of the EGR control valve 12 is controlled by feeding back the output of the opening detector 12 to obtain the EGR corresponding to the operating conditions of the engine.
In the conventional EGR control device, a large amount of carbon contained in exhaust gas will stick to the EGR control valve 12 if it is used for hours and therefore it is not controllable accurately because the initial flow rate of exhaust gas corresponding to the opening degree of the control valve changes.